Field of the Invention
Disclosed herein is a method of applying a sizing material to a fiber by use of a solvent. The method can be used to engineer the interface between the fiber and other components of a composite material into which the fiber is incorporated. In various embodiments, the method provides improved manufacturing processes and/or a composite material having improved properties.
Description of the Related Art
While carbon fibers can be used to make extremely strong products, carbon fibers themselves can be extremely fragile during processing and therefore can easily break under outside forces. To combat this fragility of the fibers, a coating can be applied to the fibers that acts as a protective barrier for preventing breakage. Typically, a material known as size or sizing is coated onto the carbon fibers to act as a protective layer and/or surface modifier. Furthermore, sizing can be used to modify the surface of the fibers, such as enhancing adhesion to the fiber of a polymer matrix. Sizing can be useful in improving the resulting composite performance. Different types of sizing can be used based on desired final qualities of the carbon fiber coating. Sizing can be applied to various types of fibers, including inorganic and organic fibers.
Historically, there have been two primary industry standard practices with regard to applying sizing to fibers (e.g., carbon fibers). First, sizing materials can be emulsified into a carrier liquid with use of a surfactant. However, use of a surfactant can negatively affect the final properties of the composite, as the surfactant may remain on the final fiber product. Moreover, many materials are incapable of being readily emulsified. For example, high molecular weight (MW) epoxies, thermoplastics, and rubbers all tend to be difficult to emulsify, and therefore are not generally used as carbon fiber sizing using the emulsification method. This leaves a substantial set of potential sizing materials that cannot be economically and/or technically used to enhance or modify the fiber surface.
A second method for applying sizing onto a fiber (e.g., carbon fiber) is through use of solvents. The sizing material can be dissolved in a specific solvent, such as an organic solvent, resulting in a solution of solvent and sizing material. Fibers can then be dipped in the solution so that the sizing/solvent solution adheres to the fibers. The fibers coated with the sizing/solvent solution are dried to remove the solvent, leaving only the sizing material on the fibers. However, the step of drying the coated fibers to remove the solvent typically produces solvent vapor, which can be hazardous to both the environment and workers exposed to the solvent vapor unless appropriate precautions are taken. For example, organic solvent vapors can be highly flammable and can catch fire and/or explode, even when substantial precautions are taken. Further, general health hazards associated with excessive solvent exposure include toxicity to the nervous system, reproductive damage, liver and kidney damage, respiratory impairment and/or cancer. Therefore, extreme care and safety costs must be taken into account when dealing with the organic vapor produced by the solvent sizing method. These precautions can make the solvent sizing method prohibitively expensive, and thus impractical.